The effects of Artemether on immune responses were investigated in mice. After p.o. administration of Artemether 200 mg/kg?d-1 for 4-7 days, it was found that: ( 1 ) the percentage of phagocytosis in peritoneal macrophage was reduced, but the phagocytic index was not significantly affected; ( 2 ) the level of serum IgG was markedly lowered in the mice sensitized by cock red blood cell ( CRBC ); ( 3 ) the hemolysin-forming capacity of CRBC-sensitized mice was significantly enhanced; ( 4 ) the weight of thymus was markedly decreased in the mice sensitized with CRBC; ( 5 ) no significant effect in PHA-induced lymphocyte transformation was found.